This invention relates to TM01 cavity resonators and to filters achieving a low insertion loss and high Q in a small size.
Coaxial cavity resonator filters and dielectric loaded single TE01 mode cavity resonators filters are two types of filter structures that have been widely used, especially in cellular-type telecommunications base stations, to provide high performance and high power handling. The typical quality factor (Q) of coaxial cavity resonators is from 2,000 to 8,000, while the Q of dielectric loaded TE01 mode cavity resonators varies from 12,000 to 40,000 when low loss, high dielectric constant ceramic materials are used. Usually, the cavity size of dielectric loaded TE01 mode cavity resonators is much greater than the size of the coaxial cavity resonators. To find a technology to fill the gap between these two technologies namely to produce a filter which has a Q greater than that of a coaxial cavity resonator filter, but which is of a size smaller than that of a TE01 coaxial cavity resonator has been a long time goal. It would be desirable to provide a dielectric loaded TE01 mode cavity resonator filter with a Q of 8000 to 12,000 without increasing the cavity size relative to coaxial cavity resonator technology, or to provide a similar Q with smaller size.
It would also be desirable to produce filters using both ceramic or metal disc loaded cavity resonators to achieve Qs in the ranges of 8,000 to 12,000 in a size smaller than is possible today when employing either coaxial cavity resonator and TE01 mode cavity resonator technologies.
In accordance with the present invention, an improved dielectric loaded cavity resonator filter is provided. The filter has at least one elongate dielectric tube resonator defining a clear through axial opening. The tube resonator is positioned in a conductive cavity such as a metallic cavity. The elongate dielectric tube resonator extends at least 70% of the height of the cavity and preferably extends substantially from the top to the bottom of the conductive cavity and has a length which is equal to or greater than its diameter. Means for securing the dielectric tube resonator in the cavity at each end of the tube resonator are provided. The securing means may comprise a mounting post at one end of the dielectric tube resonator. Desirably, the dielectric tube resonator defines centering formations in the clear-through axial opening and the centering formations engage the securing means at each end of the dielectric tube resonator. In a preferred form, the filter comprises a plurality of dielectric tube resonator/conductive cavities. The filter may also comprise a plurality of resonators, including at least one of the dielectric tube resonators and at least one coaxial resonator. The filter may also comprise tuning screws projecting into the dielectric tube resonators coaxial with the clear-through axial openings for adjusting the resonant frequency of the filter.
Also in accordance with the present invention, an improved dielectric loaded cavity resonator is provided comprising an enclosed housing defining a conductive cavity and an elongate cylindrical dielectric tube resonator defining a clear-through axial opening therein, the resonator being centrally located in the cavity and extending preferably substantially the full height of the cavity. In a most preferred form, the height of the dielectric tube resonator is equal to or greater than its diameter.